On Maria and Luke's first day at their new school,they find something oddly amiss. The new administrators all seem to have constant gastric problems,the food in the cafeteria is rotten,an unusual metallic odor in the air and the two have to contend with being in separate classes,with Maria having to deal with her opinionated classmate Clyde Langer. Sarah-Jane,for her part is curious about the new building too. When she talks to Maria's father Alen,he has been involved in a series of construction projects of the same exact model buildings all over the world-the blueprints of which contain a huge mystery room in each construct. When she goes to the school herself to investigate it turns out things have already gotten severely out of hand. Following a mysterious power outage where even Sarah-Jane's advanced equipment dies,her meeting with the head master of the school results in her becoming aware that she,along with most of the other adults at the school are menacing,green,black eyed creatures.
While trying to escape Luke,Maria and Clyde find themselves taken in by an honor student who addresses himself as a child of the Slitheen. When Sarah-Jane and the trio escape back to her home,she is able to use Mister Smith to give her more information on the Slitheen and their mission. Meanwhile the family try to use the technology Luke inadvertently gave them to not only knock out every energy source on the planet but Earth's sun as well. Sarah-Jane knows what's happened now. The Slitheen's have gained control over the Coldfire construction firm to built special facilities all over the planet near electric train depots they are using to conduct massive amounts of energy into a device which will destroy Earth's son and all life on the planet. The reason,as they explain to Sarah-Jane and her companions is due to the Doctor's successfully stopping the families attempts to invade Earth previously and their resulting misfortune. Once Clyde helps his new companions to defeat the Calcium based Slitheen family with simple vinegar,they are able to stop their plans and save the world.
It's really compelling to see Sarah-Jane Smith and her adolescent companions up against the Slitheen of Raxicoricofallapatorius, a race of self centered and insipid extra terrestrial parasites who see no value whatsoever in life on Earth,or anywhere else for that matter. Clyde Langer,while starting out as something of a class clown protagonist to Maria and Luke ends up learning the most from this experience when he's forced by necessity into helping Sarah-Jane,her son and Maria in saving the Earth from the Slitheen's plans. Even more telling in this story is Sarah-Jane's personal insecurities as a new mother. Upon finding out one of the reasons for the Slitheen attempt to destroy Earth is to steal it's resources to keep their 12 year old son from having a meaningful life,she is deeply affected by having to kill the Slitheen,especially their child. She is not able to even applaud their victory when the Earth is saved from the Slitheen. This showcases that Sarah-Jane possesses the same level of compassion as the doctor with whom she once traveled And she intends to carry that with her on her personal missions.